TOI-5205b: A Jupiter transiting an M dwarf near the Convective Boundary

We present the discovery of TOI-5205b, a transiting Jovian planet orbiting a solar metallicity M4V star, which was discovered using TESS photometry and then confirmed using a combination of precise radial velocities, ground-based photometry, spectra and speckle imaging. The host star TOI-5205 sits near the eponymous `Jao gap', which is the transition region between partially and fully-convective M dwarfs. TOI-5205b has one of the highest mass ratio for M dwarf planets with a mass ratio of almost 0.3$\%$, as it orbits a host star that is just $0.392 \pm 0.015$ $M_{\odot}$. Its planetary radius is $1.03 \pm 0.03~R_J$, while the mass is $1.08 \pm 0.06~M_J$. Additionally, the large size of the planet orbiting a small star results in a transit depth of $\sim 7\%$, making it one of the deepest transits of a confirmed exoplanet orbiting a main-sequence star. The large transit depth makes TOI-5205b a compelling target to probe its atmospheric properties, as a means of tracing the potential formation pathways. While there have been radial velocity-only discoveries of giant planets around mid M dwarfs, this is the first transiting Jupiter with a mass measurement discovered around such a low-mass host star. The high mass of TOI-5205b stretches conventional theories of planet formation and disk scaling relations that cannot easily recreate the conditions required to form such planets.